Recently, the demand for precision machined parts is increasing and lathes using various spindle devices have been put in practical use to meet the demand. A cam type lathe using such a conventional spindle device will now be described with reference to FIG. 1.
A workpiece 1 is rotated by a chuck 4 attached to the spindle 3 of a headstock 2. The torque is transmitted to the spindle by a belt 5, but the situation is that a variation in the tension of the belt 5 causes deformation of the spindle 2, making it impossible to attain accurate rotation of the workpiece 1. In an effort to avoid this difficulty, a magnetic coupling has been employed where accuracy is required, but there has been a problem that such a device is too complicated to be practical.